The present invention relates to wireless communications systems and, in particular, to power control in wireless communications systems.
Wireless communications systems use power control to improve system performance and increase system capacity. Power control involves tracking the fading of communication channels. In order to compensate for the fading, power control uses the tracked fading to manage the power level at which signals are transmitted from base stations to mobile terminals and from mobile terminals to base stations. One type of wireless communication system uses Code Division Multiple Access (CDMA) techniques.
In CDMA communication systems, digital information is encoded in an expanded bandwidth format, and multiple signals are transmitted simultaneously within the same frequency band. The number of signals that can be transmitted simultaneously is limited by the interference they cause each other. Typically, the larger the signal""s transmit power the more interference it causes other signals. Thus, reducing the power of the signals increases the capacity of the wireless communication system. However, reducing the power of a signal increases the number of errors in that signal when it is received and decoded by the receiver. A goal of power control is to keep the power level as close as possible to a level that allows the maximum capacity while keeping the number of errors in the signal at an acceptable level.
As shown in FIG. 1, when a call is set up in a CDMA wireless communications system, base station 10 and mobile terminal 20 communicate over forward link 30 and reverse link 40. Forward link 30 includes communication channels for transmitting signals from the base station to the mobile terminal, and reverse link 40 includes communication channels for transmitting signals from the mobile terminal to the base station. Base station 10 transmits certain types of control information to mobile terminal 20 over a communication channel, referred to herein as a forward control channel, also known in the art as a forward overhead channel. Forward control channels include the pilot, paging, and synchronization channels. Base station 10 transmits voice or data, and certain types of control information over a communication channel, referred to herein as a forward traffic channel. Mobile terminal 20 transmits certain types of control information to base station 10 over a communication channel, referred to herein as a reverse control channel, and it transmits voice or data over a communication channel, referred to herein as a reverse traffic channel. The signals on the communication channels are organized in time periods, referred to herein as frames. Frames are typically 20-millisecond (ms) in length. The signals transmitted over the control channels are referred to herein as control signals, and the signals transmitted over the traffic channels are referred to herein as traffic signals. Forward traffic frames are frames transmitted over the forward traffic channel, and reverse traffic frames are frames transmitted over the reverse traffic channel. Each forward and reverse traffic frame includes voice or data and error control information, typically in the form of a cyclical redundancy code (CRC).
Power control varies the power output of base station 10 and mobile terminal 20 to maintain a constant frame error rate at both the base station and the mobile terminal. A frame error occurs when one or more uncorrectable bit errors occur in a frame. The frame error rate is the number of frame errors divided by the total number of frames observed. A desired frame error rate is selected to minimize power and therefore optimize capacity without compromising signal quality. If the frame error rate exceeds the desired frame error rate, the usefulness of the signal is reduced and the power level is increased to decrease the number of frame errors. If the frame error rate is below the desired frame error rate, the power level exceeds the optimum power level, and the power level is reduced.
In CDMA 2000 wireless communications systems, the power control information in updated at an 800 Hz rate on both the forward and reverse links. Each frame includes sixteen 1.25 ms time intervals, referred to herein as power control groups. Power control information, referred to herein as a power-control bit, is sent once every power control group, or every 1.25 ms.
In CDMA 2000 communication systems, power control on the reverse link is implemented using outer loop 50 and inner loop 60. Outer loop 50 adjusts a targeted signal-to-noise ratio for the reverse link, where the targeted signal-to-noise ratio is chosen to produce a desired frame error rate. Inner loop 60 keeps the signal-to-noise ratio on the reverse link as close as possible to the targeted signal-to-noise ratio. Signal-to-noise ratios are often expressed as the ratio Eb/N0, where Eb is the energy per information bit and N0 is the power spectral density of the interference seen by the receiver.
Outer loop 50 of base station 10 determines targeted Eb/N0 70 using a desired frame error rate, which is typically 1%, but can be increased or decreased depending on the desired system performance. In outer loop 50, base station 10 checks the CRC of each reverse traffic frame to determine whether the reverse traffic frame contains an error. If there is an error in the reverse traffic frame, targeted Eb/N0 70 is increased by one up step size. If there is no error in the reverse traffic frame, targeted Eb/N0 70 is decreased by one down step size. The down step size is typically much smaller than the up step size. For example, in a typical system, the down step size is about 0.01 dB, and the up step size is about 1 dB. The ratio of the down step size to the up step size is set equal to the desired frame error rate. For example, 0.01 dB/1 dB=1%, so that in steady state, the targeted Eb/N0 70 settles at a value close to the Eb/N0 needed to achieve the desired frame error rate.
In inner loop 60, targeted Eb/N0 70 is compared to the received signal""s Eb/N0 80 at the end of every power control group. The base station measures and averages the energy per information bit for the power control group, and it measures and averages the noise and interference of the signal for the power control group. The ratio of these two averages is power-control-group (pcg) Eb/N0 80. Although, the pcg Eb/N0 80 can be measured in any way that obtains an accurate measurement. When pcg Eb/N0 80 is smaller than targeted Eb/N0 70, base station 10 sends a power-control bit on forward link 30 indicating that mobile terminal 20 should increase the power of reverse link 40 by a fixed amount. When pcg Eb/N0 80 is larger than targeted Eb/N0 70, base station 10 sends the power-control bit on forward link 30 indicating that mobile terminal 20 should decrease the power of reverse link 40 by a fixed amount.
In some conventional CDMA wireless communications systems, the reverse-link power control is identical to the reverse-link power control for CDMA 2000 wireless communications systems described above. In other conventional CDMA systems, the reverse-link power control is slightly different. In the latter systems, instead of measuring the Eb/N0 for every power control group, the base station measures a different energy measurement closely related to the Eb/N0 and uses this energy measurement instead of Eb/N0.
The forward-link power control in some CDMA 2000 systems also works similarly to the reverse-link power control in CDMA 2000 systems described above. In outer loop 110, mobile terminal 20 determines targeted Eb/N0 120 using a desired frame error rate, which is typically 1%, but can be increased or decreased depending on the desired system performance. In outer loop 110, mobile terminal 10 checks the CRC of each forward traffic frame to determine whether the forward traffic frame contains an error. If there is an error in the forward traffic frame, mobile terminal 20 increases targeted Eb/N0 120 by one up step size. If there is no error in the forward traffic frame, mobile terminal 20 decreases targeted Eb/N0 120 by one down step size. In inner loop 140, targeted Eb/N0 120 is compared to pcg Eb/N0 150 of the received signal, measured by the mobile terminal 20""s receiver. When pcg Eb/N0 150 is smaller than targeted Eb/N0 120, mobile terminal 20 sends a power-control bit on reverse link 40 indicating that base station 10 should increase the power of forward link 30. When pcg Eb/N0 150 is larger than targeted Eb/N0 120, mobile terminal 20 sends a power-control bit on reverse link 40 indicating that base station 10 should decrease the power of forward link 30.
A problem with this power control system is that power adjustments are not fast enough to compensate for changes in communication channel characteristics produced by fast moving mobile terminals. The above-described system adjusts the targeted Eb/N0 once per frame based on whether the frame is an errored frame. This technique may result in a long delay before the targeted Eb/N0 is adjusted to a value that will produce the desired frame error rate, which is typically 1%. Several frame errors must be observed before the actual frame error rate may be accurately determined. This means that hundreds, or possibly even thousands, of frames are needed before the frame error rate reaches the targeted percentage. However, during these frames, while the power control loop is still adjusting the transmitted power to get to the desired frame error rate, the propagation losses between the mobile terminal and the base station can vary due to movement of the mobile terminal. The change in the propagation losses can cause a change in the Eb/N0 needed to obtain the desired frame error rate. Thus, the required Eb/N0 needed to get the desired frame error rate can change in less time than it takes for the reception of the number of frames needed to adjust the frame error rate to the targeted percentage. This produces one of two problems. In one problem, too much power is transmitted, causing a reduction in capacity of the wireless communication system, which reduces the revenue generated by the system. In the other problem, not enough power is transmitted, causing an increase in the number of errors, which reduces the usefulness of the signal.
The invention solves the above problems by allowing for quicker adjustments of a targeted signal quality measurement in a wireless communication system through the use of an effective signal quality measurement for each time period. The signal quality measurements of the time period produce a certain quality of service measurement. The quality of service measurement is a measurement of how well the signal is received. For example, the quality of service measurement can be the frame error rate. The effective signal quality measurement is the signal quality measurement that would produce this certain frame error rate in a model channel. The model channel can be any channel where the signal quality measurement varies in a well-defined and known manner over time. For example, the model channel can be an additive white Gaussian noise (AWGN) channel, in which the signal quality measurement remains constant over time. Obtaining an effective signal quality measurement for each time period allows quicker adjustments of the targeted signal quality measurement. This permits the targeted signal quality measurement to be closer to a signal quality measurement that produces a desired frame error rate. This allows the transmitted power to be closer to the power needed to produce the desired frame error rate, permitting an increase in the capacity of the wireless communication system while still maintaining an acceptable number of errors.
The system implements power control by obtaining the effective signal quality measurement for each time period, comparing it to a model targeted signal quality measurement, and adjusting the targeted signal quality measurement based on the comparison. The effective signal quality measurement for the time period is obtained by measuring a signal quality measurement for each interval of a time period to obtain a plurality of signal quality measurements for a time period and mapping these signal quality measurements to obtain the effective signal quality measurement. For example, the effective signal quality measurement for the time period can be obtained by mapping a vector signal quality measurement into a scalar. A signal quality measurement is measured for each interval of a time period. The signal quality measurements for all of the intervals in one time period compose the vector signal quality measurement.
The model targeted signal quality measurement is the signal quality measurement value that produces a desired frame error rate in the model channel. The effective signal quality measurement is then compared to the model targeted signal quality measurement. The targeted signal quality measurement is increased by an up step size when the effective signal quality measurement is smaller than the model targeted signal quality measurement, and decreased by a down step size when the effective signal quality measurement is larger than the model targeted signal quality measurement.
The wireless communication system includes a transmitter to transmit a signal, a receiver to receive the signal, and an outer control loop to control targeted signal quality measurement. The outer control loop controls the targeted signal quality measurement by adjusting it based on a comparison of the effective signal quality measurement of the signal to the model targeted signal quality measurement. In one embodiment, the invention is implemented in software; in another embodiment, the invention is implemented in hardware. In the latter embodiment, the system has a signal quality measurement detector for measuring a signal quality measurement for an interval of a particular time period. The outer control loop has a memory for storing a plurality of signal quality measurements. An output of the memory is coupled to a processor that obtains an effective signal quality measurement. An output of the processor is coupled to a first comparator which compares the effective signal quality measurement to the model targeted signal quality measurement. An output of the first comparator is coupled to a control input of a summer. The summer increases the targeted signal quality measurement by an up step size when the effective signal quality measurement is smaller than the model targeted signal quality measurement; and it decreases the targeted signal quality measurement by a down step size when the effective signal quality measurement is larger than the model targeted signal quality measurement.